For source tubes for bearing element parts such as races, shafts, rollers, needles and balls, high Cr (chromium) bearing steel such as SUJ2, specified in JIS G 4805 Standard is widely used in general.
The above, the so-called “bearing steel”, is subjected to processing by means of hot rolling and the like, and then to spheroidizing annealing for the purpose of softening, followed by processing such as cold rolling, cold drawing, cold forging and machining, which finally undergoes the heat treatment comprising quenching and tempering at low temperature, thus resulting in having desired mechanical properties.
Since, among the above processing steps, the machining step is costly, there is a growing demand for bearing steel having excellent machinability that will enable to enhance cutting-machining efficiency and extend tool life.
A free-cutting metal element (a metal element to act for enhancing machinability) such as Pb and S is well known to improve machinability when it is added independently or in combination with the other(s). However, bearings to be used for industrial machineries, automobiles or the like are subjected to repetitively high surface pressure. In this regard, the addition of above free-cutting element(s) will cause a fatigue life in rolling contact to substantially decrease.
Furthermore, the above free-cutting metal element generally causes hot workability to decrease, thereby becoming more of an issue that surface cracking and defects are likely to be generated during hot working process such as hot rolling for bearing steels.
For example, in Japanese Patent Application Publication No. 01-255651, high Si- low Cr bearing steel having excellent machinability, which contains REM (rare-earth metals) in chemical compositions, is disclosed. However, since REM is very likely to be oxidized, a yield in steel making is unstable and it is difficult in commercial operation to control the globular size of REM oxides and the state of dispersion morphology of the same, whereby the coarse REM oxides tend to be generated, and the generation of a large amount of REM oxides leads up to a substantial decrease of the fatigue life in rolling contact.
In Japanese Patent Application Publication No. 03-56641, a bearing steel having excellent machinability, which contains BN compounds in the chemical compositions that enhance the machinability without decreasing the fatigue life in rolling contact, is disclosed. However, since B has little solubility in the steel, its yield is unstable in the steel and its segregation is likely to be generated. Further, B causes a solidification startup temperature to markedly decrease, thereby ending up promoting a solidification segregation in association with B segregation. In addition, the decrease of solidification startup temperature amounts to a reduction of hot workability, thereby ending up in likely generation of surface cracking and defects during hot working process.
Accordingly, the bearing steel having a B content which is specified in the above Japanese Patent Application Publication No. 03-56641, namely 0.004-0.020% in weight, is not commercially and reliably processed to be bearing element parts.
In Japanese Patent No. 3245045, a bearing steel having excellent machinability as well as cold workability and a method for manufacturing the same is disclosed, wherein the number of carbides in the metal structure and the hardness are adjusted by the heat treatment to be applied under specific condition. However, in this annealing condition by above invention, a slow heating or an isothermally holding in its heat-up step is required. In this regard, the annealing time becomes longer, thus reducing the productivity.
Furthermore, since, in a continuous heat treatment furnace which is commercially used, the temperature in each zone is predetermined and the number of zones is limited either, it is difficult to perform annealing under the condition specified in above Japanese Patent No. 3245045, whereby restructuring or renewal of the continuous heat treatment furnace becomes necessary in order to carry out annealing under the specified condition, thus ending up in a cost increase.
The disclosed technologies above could at least provide a bearing steel tube having excellent machinability to be used for bearing element parts. But, as afore-mentioned, the productivity as well as quality will potentially become more of an issue.